1. Technical Field
The presently disclosed technologies are directed to a system and method for reducing print quality defects due to excursions in the velocity of media transport caused by a transfix nip in a direct marking printing system. The system and method described herein use sensors to measure the media thickness and/or skew to adjust the transport speed and decrease potential print quality defects.
2. Brief Discussion of Related Art
In order to ensure good print quality in direct marking printing systems, the velocity of the media transported through the printing system must remain at a predetermined rate. Typically, a printing system is designed for media with a specific thickness. The design anticipates the change in velocity that occurs as the media is transported through the system and compensates for them. However, when media having a different thickness is used, the transport velocity can increase or decrease and can cause print quality defects.
A conventional transfix nip consists of an imaging drum and a transfix roll which is preloaded against the imaging drum. Media entering this nip cause a velocity transient which results in a degradation of image quality. U.S. Pat. No. 7,065,308 describes a system to reduce velocity transients using feed forward torque control to a nip motor. When the media enters the nip, the motor speed changes based on media characteristics obtained from table entries. This requires a priori knowledge and is also subject to operator error when specifying the correct media type. These errors can result in a compromised capability of the feedforward control and resulting image degradation.
The prior art includes various methods for reducing motion disturbances caused by feeding sheets into a transfix nip. The nip is formed between an imaging drum and a transfix roll under a pre-load which forces them together. Both the imaging roll and the transfix roll are driven by a servo motor. Sheets are fed into the nip causing motion disturbances. One method in the prior art for reducing motion disturbances supplies a supplemental (i.e., feedforward) torque profile to the transfix roll upon sheet arrival at the interface. A table based upon the media characteristics is used to determine the transfer roll drive current that must be supplied to increase and decrease the imaging drum rotational velocity in order to maintain a substantially constant imaging drum rotational velocity.
One prior art method reduces the torque disturbance of the lead and trail edge of a sheet by applying a supplementary (i.e., feedforward) torque profile to the imaging drum when the sheet arrives or leaves the nip formed by the imaging drum/transfix interface. Another prior art method uses the effect of skew to reduce the velocity transient. In this method, the transfix roll is skewed relative to the imaging drum. This is similar to the method that has no skew between the imaging drum and the transfix roll but the media entering the nip is skewed. Still another method measures the sheet entry torque spike at an upstream media path nip to control the torque supplied to a downstream media path nip to counter-act “thump”—the velocity transient resulting from the nip roller engaging the sheet. Accordingly, there is a need for a system and a method that reduces the velocity transient caused when the nip roller engages the media.